A magnetic recording medium typically consists of a magnetizable material affixed to a substrate. One general type of magnetic medium consists of a magnetizable cured pigment binder coating, derived from a dispersion of magnetizable pigment particles in a polymeric binder, which is coated on a flexible, typically polymeric, non-magnetic support.
Magnetic media are provided in a variety of forms, including audio and video tapes, computer diskettes, and stretched surface recording disks (SSR). SSR generally consist of a rigid, circular support and a thin polymeric film, having a recording layer, suitably attached to the periphery of the support. See, for example, U.S. Pat. No. 4,623,570.
For optimum performance, magnetic media should have exceptionally smooth, durable recording surfaces, with a minimal coefficient of friction. In present recording media the coefficient of friction often changes with changes in ambient humidity. This causes unwanted variations in the performance of the media in varied use environments.
Lubricants are often used in magnetic media to aid in minimizing the wear between the media and the recording head. A lubricant is either added to the dispersion of binder and magnetizable particles before coating, or added as an overcoat layer and allowed to soak into the magnetic layer.
A successful magnetic medium will have a sufficient surface conductivity to avoid electrical static charging during use. For example, a nonconductive magnetic layer coated on a non-conductive flexible substrate, such as polyethylene terephthalate (PET) or polyamide, may require some special treatment to attain adequate conductivity. This may be accomplished by adding a conductor, such as carbon black, to the magnetic pigment before coating. As an alternative, a non-magnetic carbon black coating may be coated onto the back side of the flexible substrate, or underneath the magnetic layer on the front side of the medium.
Each of these methods of providing conductivity has deficiencies. Where carbon black is added to the magnetic layer signal loss results, since the carbon black occupies space that could be used for magnetic pigment. A separate carbon black layer requires extra processing steps, and is therefore costly and decreases production yield.
In present magnetic media, water vapor can permeate the magnetic layer and cause degradation of durability. A magnetic layer that is excessively permeable to water vapor and oxygen can also permit fungal growth or oxidation of the magnetic pigment.
Carbon coatings have been applied to rigid disks and non-binder type magnetic media, see, e.g., U.S. Pat. No. 4,833,031, but have not been used to successfully address durability, permeability, nor the stability of the coefficient of friction with changes in ambient humidity. Flexible magnetic recording media typically address durability by utilizing surface applied lubricants, and use carbon black to enhance conductivity.
Means for preparing a carbon coating by plasma deposition have existed previously, but have had deficiencies. U.S. Pat. No. 4,645,977 shows a method for forming a carbon film through the use of an induction plasma. The use of high gas flow, pressure, and power can cause formation of carbon powder, instead of the desirable smooth, hard carbon film.
Despite the prior availability of carbon coatings, and the means for plasma deposition of carbon coatings, there remains a need for a flexible magnetic medium having a polymeric layer containing magnetizable pigment, that does not require conductive particles dispersed in the binder layer or in a separate conductive layer. There is also a need for a magnetic medium that is very durable and has a low coefficient of friction that is stable throughout a range of ambient humidities ranging from 0 to 100%. There is also a need for a magnetic recording medium that is substantially impervious to water vapor and oxygen.